strictly speaking you can say MPE is unnecessary, Mode 4 is 'nearly' there (and Ive seen panel discussions where the 'midi pioneers' have argued this)
but if you stand back, and say you dont want to musicians/consumers to have to worry about midi protocols this is harder to defend.
Ive had an Eigenharp before MPE days, and at that time, I had to have various configurations to work with different synths, due to different PB ranges, or receiving 'timbre' on different CCs, using poly pressure, or channel pressure - the Eigenharp is great at this because EigenD its software has a massive configuration matrix. (see below for screenshots of a small part of the config) so was flexible for every situation, it worked (still does) , but frankly it was a pain when you just wanted to play
do you want that level of complexity for all users? and this is nigh impossible for a strict hardware only device.
so MPE simplifies, +/-48 PBR, CC 74 , channel pressure, you just set MPE mode and off you go.
(number of voices and PB range are sent automatically from controller to sound generator)
mode 4, is strictly monophonic voices, MPE allows for polyphony on 1 channel (e.g. use 8 midi channels, but allow multiple voices of each channel ... ok, you loose note independence but its allowed)
global channel - EigenD allowed for this too, but for many synths, they didn't understand this, this meant something like a 'breath control' would have to transmit the same data on all channels (so redundant data).
mode 4, doesn't have any provision for splits
I also think some of it is 'marketing', especially since Roli got into the driving seat on the spec. the manufactures needed a way to communicate what expressive midi was, and what it required of synths... i.e. if you say you synth/VST is MPE compatible, it implies you can just plug it into your Seaboard/Soundplane/Contrinuum/Eigenharp etc.
Im not saying MPE is perfect, it definitely has its flaws... and who knows if the MMA will ever ratify it, but at least its something to 'rally behind'
btw: Id point out the newer 1.25a MPE spec - its details have changed quite a bit from the original spec, that many saw.